An analytical method for estimating voiding rate in a flow stagnated core during a constant pressure heatup process

In a sudden and complete loss of flow accident of a nuclear reactor, the reactor coolant flow will become stagnant almost instantaneously. As a result, the coolant is heated up and voids are generated. The knowledge of voiding rate is quite important for determining flow pattern, heat transfer, cladding temperature and reactivity feedback predictions. Usually, the voiding rate is calculated from large and complex computer codes. In this study, a simplified analytical method is derived for estimating the voiding rate of a core with initial stagnant flow assuming the transient to be a constant pressure process. Although there are restrictions in the application range, this method was developed to be a quick scoping method, which is simple enough for hand calculation. This analytical method has been compared with RELAP5/MOD2 calculations with good results. A practical application of the method to the Taiwan Research Reactor is presented which illustrates the usefulness of this method in providing assistance to engineering judgement in safety analysis. (orig.)